Occurrence of Minerals in Coal and Its Geological Controlling Factors—Significance in Evaluation of Coal Washability

Abstract

The precombustion washing and washability of coal are increasingly gaining attention as a clean coal technology in China. In this study, the raw coal was collected from different coal seams in the south of Qinshui Coalfield, and flotation experiments were carried out in the laboratory after simple crushing. X-ray diffraction was used for qualitative analysis of the minerals in coal, and mineral morphology and disseminated particle size were measured with the help of an optical microscope. The samples were characterized to determine the original mineral content, combination type, occurrence characteristics, and dissemination type in coals of different lithotypes to establish a relationship with the coal-forming environment and to evaluate coal selectivity from a geological perspective. The results were obtained from the float–sink experiment by which the coal washability of different lithotypes is evaluated, combined with the information resulting from investigating the change law of the mineral occurrence and dissemination characteristics of the raw coal and the clean coal before and after the flotation experiment. The content of minerals with the particle size larger than 200 μm accounts for 35–50% of the easy-to-wash coal. The particles were mostly concentrated in the 50–100 μm size range in moderate difficulty washability coal, and the mineral occurrence forms are diverse. The disseminated particle size distribution of 10–20 μm in the refractory washability coal is between 35 and 45%, and the massive and fracture filling minerals are rarely found in the coal. The proportion of minerals smaller than 2 μm in the extremely difficult washability coal is more than 60%, and cell filling, dispersed, and banding forms are plentiful in this type of coal. The removal rates of ash and sulfur obtained through elution match well with the mineral form and distribution characteristics. The reason for the difference was the influence of the coal-forming environment on the occurrence of minerals in coal, which led to the removal of minerals in the coal washing procedure. The process is controlled by the occurrence and dissemination characteristics of minerals.

Occurrence Characteristics of Fine-Grained Pyrite in Coal and Its Scaling Effect on Flotation Desulfurization

“…Aggregates of pyrites with different forms present various macroscopic and microscopic morphologies. As pyrites smaller than 60 μm are difficult to remove directly, 60 μm is the boundary between fine-grained particles and coarse particles of pyrite according to the classification standard of Yang . The microscopic morphology of pyrites in Guizhou and Shanxi coal samples was observed by SEM, and the particle sizes of pyrites with different microscopic morphologies were measured and are shown in Table .…”

“…As pyrites smaller than 60 μm are difficult to remove directly, 60 μm is the boundary between fine-grained particles and coarse particles of pyrite according to the classification standard of Yang. 17 The microscopic morphology of pyrites in Guizhou and Shanxi coal samples was observed by SEM, and the particle sizes of pyrites with different microscopic morphologies were measured and are shown in Table 3 . Among them, the aggregated massive pyrite has a larger particle size, while the particle sizes of the framboïdal, euhedral crystalline, and agglomerated pyrites are generally less than 60 μm, and the morphology of the filled pyrite (fracture-filling pyrite and biostructured pyrite) is controlled by the filling location.…”

“…However, fine-grained pyrites (smaller than 60 μm) are difficult to separate by the flotation method. − On the one hand, the formed foam of the mixture of fine-grained pyrite and coal particles during the flotation process has strong stability and low fluidity, making the pyrite difficult to separate from coal . On the other hand, the fine-grained pyrite can form a complex symbiotic relationship with the coal matrix, , resulting in a flotation behavior similar to that of coal particles.…”

“…On the other hand, the fine-grained pyrite can form a complex symbiotic relationship with the coal matrix, , resulting in a flotation behavior similar to that of coal particles. In addition, the sulfur atoms on the crystal surface of fine-grained pyrite are easily replaced by carbon atoms, ,, reducing the physical adsorption ability of pyrite toward water molecules and increasing the difficulty of separation. Wang et al analyzed the influence of the coal particle size on flotation recovery and concluded that the optimum coal particle size ranges from 50 to 250 μm (recent advances in the beneficiation of ultrafine coal particles) .…”